In the distribution of electrical energy, electric utility companies have typically found it desirable to measure not only real load energy as watthours delivered to a user, but also, reactive load quadergy as varhours (or reactive volt-ampere hours), apparent power and power factor. By measuring both watthours and varhours, electric utilities can more accurately apportion the costs of supplying energy to customers having varying demands and loads. To facilitate this goal, electrical utilities have sought to replace conventional electromechanical meters with more accurate and reliable microprocessor-controlled solid state meters which can measure both real and reactive quantities.
One such meter is the General Electric Type EV/ES Electronic Demand Meter with a TM-900E.TM. Time-of-Use (TOU) Register. This meter, which is commercially available from General Electric Company of Somersworth, N.H., includes solid state metering and registering circuits therein and has proven to be an industry leader because of its reliability, measurement accuracy and cost. The measurement circuit of this meter contains a measurement integrated circuit (IC). A potentiometer and a binary-coded rotary switch are also provided for facilitating factory and field calibration of the measurement IC. As understood by those skilled in the art, the potentiometer adjusts the full load gain of the measurement IC and the binary-coded rotary switch adjusts the non-unity power factor response of the measurement IC. The potentiometer and rotary switch insure that both real load energy and reactive load quadergy are measured accurately. FIG. 1 illustrates a functional block diagram of this prior art meter having a "full load adjust" circuit which includes a potentiometer and a "power factor adjust" circuit which includes a rotary switch. This prior art meter is more fully described in an instruction manual No. GEH-5071 entitled, Type TM-900E.TM. TOU Register and Type TMR-900E.TM. TOU Recording Register On Type EV/ES Meter Platform, which is publicly available from the General Electric Company, Somersworth, N.H., the disclosure of which is hereby incorporated herein by reference.
Full load adjustment of this meter is typically accomplished by a pre-calibration step and a final calibration step. During the pre-calibration step, a fixed value resistor is soldered into the full load adjust circuit after its value is determined from the results of an accuracy test performed on the measurement circuit. Then, during subsequent calibration steps, the potentiometer is adjusted until the meter is calibrated to within a few hundredths of a percent of 100% registration. The non-unity power factor adjustment step, which is performed after the full load calibration step, includes the steps of turning a 16-position binary-coded switch (BITS 0-3) until the "lag" registration is within 0.1% of the full load registration. Both the full load and lag final calibration adjustment steps are performed by trial and error. Typically, a calibration reading is taken manually or by a computer and then a test operator turns the potentiometer or switch in the appropriate direction so that the next calibration reading is within final limits. If not, another calibration step is performed and then repeated, if necessary, until proper registration is achieved. Besides the obvious expenses associated with this form of manual calibration, the potentiometer is also susceptible to "creep", which means that the resistance of the potentiometer can shift to higher or lower values during installation and/or during the lifetime of the meter. In addition, both the potentiometer and binary-coded rotary switch are relatively expensive.
One attempt to prevent potentiometer "creep" is disclosed in U.S. Pat. No. 4,843,306 to Meuller. In particular, Meuller discloses a meter having a pair of calibration resistors which can be removed and replaced with other resistors during initial and field calibration. However, in order to calibrate the meter, repeated "trial and error" calibration steps may be necessary to achieve final calibration, which can be time consuming and expensive as explained above.
Thus, notwithstanding these prior art meters, there still exists a need for an electrical energy meter of low cost design which can be simply and accurately calibrated.